In this study, we investigated the effect of multiple oral dosing of ketoconazole (KTZ) on pharmacokinetics of quinidine (QN), a CYP3A substrate with low hepatic clearance, after i.v. and oral administration in beagle dogs. Four dogs were given p.o. KTZ for 20 days (200 mg, b.i.d.). QN was administered either i.v. (1 mg/kg) or p.o. (100 mg) 10 and 20 days before the KTZ treatment and 10 and 20 days after start of KTZ treatment. Multiple oral dosing of KTZ decreased significantly alpha and beta, whereas increased t(1/2beta), V(1), and k(a). The KTZ treatment also decreased significantly both total body clearance (Cl(tot)) and oral clearance (Cl(oral)). No significant change in bioavailability was observed in the presence of KTZ. Co-administration of KTZ increased C(max) of QN to about 1.5-fold. Mean resident time after i.v. administration (MRT(i.v.)), and after oral administration (MRT(p.o.)) of QN were prolonged to about twofold, whereas mean absorption time (MAT) was decreased to 50%. Volume of distribution at steady state (V(d(ss))) of QN was unchanged in the presence of KTZ. These alterations may be because of a decrease in metabolism of QN by inhibition of KTZ on hepatic CYP3A activity. In conclusion, multiple oral dosing of KTZ affected largely pharmacokinetics of QN after i.v. and oral administration in beagle dogs. Therefore, KTZ at a clinical dosing regimen may markedly change the pharmacokinetics of drugs primarily metabolized by CYP3A with low hepatic clearance in dogs. In clinical use, much attention should be paid to concomitant administration of KTZ with the drug when given either p.o. or i.v.